Testing and characterization of optical fiber during the course of its installation is an important step in the process of deploying an optical communication system. Measurements such as fiber length, loss and chromatic dispersion have generally been performed manually by the personnel installing new fiber, recording this information to create a “start of life” characteristic profile for a given span of fiber. In some cases, this characteristic profile is used to select the proper operating values for parameters of optical devices deployed along the fiber span. Given the ever-increasing desire for more centralized control of optical communication systems (through network management techniques such as Software Defined Network (SDN) concepts), the ability to automate the characteristic profile measurement process is becoming important. Indeed, the desire is to be able to continuously update the profile information on every fiber span in a system so that intelligent routing decisions can be made, providing dynamic control within the network. With both cost and size constraints being a concern, the ability to embed this type of metrological analysis into existing functions is essential.
There is also an increase in the deployment of distributed Raman amplification within optical communication networks to allow for higher data rate transmission to remain error-free. Inasmuch as parameters associated with the Raman sources are a function of the fiber's characteristics, the ability to measure loss characteristics of a given fiber span, as well as components disposed along a fiber span, is becoming critical to obtaining optimum Raman amplifier performance. Identification of loss and reflection events, such as high loss bends, poor connector quality, or splices can point out possible failure modes before there is an interruption in service. “Live” (i.e., real-time) loss measurements can enable identification of failing networks before catastrophic damage occurs. Additionally, when fault locations can be quickly and accurately identified, the down-time for the affected fiber span is minimized. Optical budgets can be determined (and controlled) by providing information on the best and most efficient routes to direct data channels.
For many years, conventional OTDR instruments have been used to characterize optical fiber spans. The basic OTDR technique transmits high-power, short laser pulses (also referred to as “probe pulses”) along the fiber span being measured. Any light that is then backscattered (Rayleigh scattered) or reflected (Fresnel reflection) in the reverse direction along the fiber is captured by a photodetector component of the OTDR, with a temporal and amplitude analysis of the return signal providing a characterization profile of the fiber span. Narrow pulse width provides high spatial resolution for loss and reflection events. However, as the returned signal is proportional to the energy contained in the pulse, the use of narrow pulse widths also results in reduced strength of the received signal. For example, a pulse width of 10 ns provides a spatial resolution of one meter, and returns only 0.0000001 of the transmitted power as Rayleigh backscatter. This low power is further reduced by the two-way loss introduced by the fiber. Thus, fiber span measurement distance accuracy is primarily defined by the width of the probe pulse, where shorter pulse widths provide less data in the return signal but provide more accurate span length results.
Additionally, since the OTDR system itself has a limited bandwidth, the fall time of the probe pulse is not infinitely fast. Therefore, if there are two reflective events spaced very close together along a fiber span, it is possible that the second event may be “missed” by the system if the signal associated with the first event has not dropped appreciably by the time the second event occurs. That is to say, when two reflections are spaced closer together than this limit, they essentially become indistinguishable. This is defined as the event dead zone. Another related parameter is defined as the attenuation dead zone, where a return signal from one event temporarily saturates the optical detector, creating a period of time where the detector cannot accurately perceive a second event. The problems with these “dead zones” in OTDR systems have been known about for years, with various types of work-arounds proposed.
One prior art technique for addressing these limitations of conventional OTDR systems relates to the utilization of a specific coding scheme in the probe pulse train. The use of a coded pulse stream allows for the pulse width of each individual pulse to remain relatively short, yet longer spans of fiber are able to be accurately characterized. However, these benefits come at the price of requiring highly complex software to generate and then process the coded OTDR waveforms, with impacts on the measurement time (as well as requiring dedicated laser sources to provide accurate input probe pulse data codes).